The parent patent application referenced above describes an automatic apparatus to provide refined positions which improve the accuracy of the digital readouts of a remote CNC (computer numeric controlled) machine tool table. In order to obtain the best accuracy, the exact directions of the X, Y, and Z axes associated with these digital readouts must be determined. An embodiment of an apparatus and method for generating and measuring machine tool table directions relative to a set of mutually orthogonal axes, and hence a first refinement to automatically improve the accuracy of X, Y, and Z digital readout positions of the remote CNC machine tool table, is disclosed herein.
After the set of mutually orthogonal axes are determined the best accuracy for the refined positions, automatically corrected for environmental factors as described in the parent patent application, is thereby defined as the second refinement. Also described in the parent patent application is a remote numeric control of the table motions and display of X, Y and Z raw and refined positions.
An axis is characterized as a direction in space with uniform increments located along that direction. The relevant axis directions for the vise, the spindle, and the table are described next.
Shown in the top view of FIG. 1A is the machine tool table 101 with vise fixed jaw 102 and vise clamping jaw 103 holding a part 100. The fixed vise jaw 102 has a reference surface with points 120 and 121 to define the vise X axis direction 140. The fixed vise jaw 102 reference surface also determines the vise Y axis direction 141. The table 101 has motion X axis direction 104, and motion Y axis direction 105. The machine tool table 101 may move in an X axis direction 104 different from the vise reference surface X axis direction 140. Also shown in FIG. 1A are tool holder 111, spindle 110, and measuring probe 112 as discussed below.
FIG. 1B is a front view of the table direction (but not actual table motion) Z axis direction 106 as defined by the spindle 110 which moves relative to the table 101. The vise fixed jaw 102 has a reference surface that also defines the vise Z axis direction 142 as shown. The vise Z axis is suitable for determining the table Z axis direction 142, and is defined by the vise as shown in the FIG. 1B front view when the vise is clamped to the table 101, as is standard machine shop practice.
Also shown in the FIG. 1B is the Z axis 106 for spindle 110 holding a tool holder 111 and measuring probe 112. The spindle 110, tool holder 111 and measuring probe 112 are rotated about and translated in the Z axis direction 106. The spindle Z axis direction 106 may be different from the table Z axis direction 142.
Typically raw X, Y, and Z positions of reference surfaces may be measured by a probe 112 and displayed by X, Y, and Z digital readouts 160 shown in FIG. 1B, and also described in the parent patent application. These probes typically measure relative position, such as between two reference surfaces on or related to the table 101, and may not determine the X or Y spindle location of Z axis direction 106. As described in the parent patent application, electronic gauge blocks located on the table 101 (not shown in FIG. 1A) utilizing a probe held in a shrink fit tool holder 111, are useful to automatically determine the X, Y and Z spindle location of Z axis direction 106 relative to a reference surface on each of the X, Y and Z electronic gauge blocks mounted on table 101. The Z axis reference surface 119 for the location of positions in Z axis direction 106 is shown on the tool holder 111.
An example of the measuring probe 112 is made by the company Swiss Precision Instruments (SPI) and is sold as part number 98-316-3. The manually operated SPI measuring probe 112 has a light that turns on when the cylinder measuring tip touches an X or Y or Z edge. A benefit of the SPI measuring probe 112 is that when the measurement is taking place, zero force is applied to the surface; hence it does not disturb the measurement of delicate parts. The measuring probe 112 may have a cylindrical tip 113 with a well-known diameter or geometrical offset 122 from the spindle Z axis direction 520, as well as a well-known geometrical offset 122 from the Z axis reference surface 119.
Tool holder 111 allows the CNC mill to automatically change the measuring probe 112, for example back and forth to another tool holder identical to 111 that may hold, for example, the heat shrink mounted probe not shown here but described in the parent patent application.
An orthogonal coordinate system comprises X, Y, and Z axes, and is useful to describe the working space where a part 100 is fabricated above the machine tool table 101. Presently the non-orthogonal coordinate system, and associated X, Y and Z axis digital readouts 160, for typical machine tool tables is accurate to within ±0.002 inches. Such X, Y and Z axis digital readouts 160 are often not trusted by the machinist. The practice of final inspection after fabrication of the part 100 is trusted to better than ±0.00005 inch (±50 pinch) using instrumentation and an orthogonal coordinate system separate from the CNC mill.
Special inspection tolerance of ±1 pinch for a gauge block is possible which we refer to as a high precision grade. Gauge blocks are not shown herein, but are described in the parent patent.
It is desirable to have accuracy of ±0.0002 inches for an orthogonal machine tool table coordinate system, and associated X, Y and Z axis digital readouts 160, which determine the mutually orthogonal X and Y axis positions of machine tool table 101, and Z axis position of tool holder 111.
The table 101 typically has jack screws on the floor (not shown) to level the table 101 by tilting the Z axis direction 142 of table 101, also resulting in tilting of the attached gantry structure (not shown) holding spindle 110 to the supporting structure for table 101. Such tilting by the jack screws, or inadvertently by the floor moving, may disturb the alignment between the Z axis direction 106 of the spindle 110 relative to the table Z axis direction 142 because the gantry structure is not perfectly rigid.
It is desirable for the motion of the spindle 110 holding and rotating the tool 112 in the Z axis direction 142 to be determined as aligned nearly parallel in the same direction relative to the table Z axis direction 106 so as to achieve high accuracy.
From the machinist's perspective, the definition of the X, Y, and Z axes is first described by an engineering drawing for the part 100. The engineering drawing provides information as to the linear and angular tolerances required for features fabricated into part 100. The X, Y, and Z axes are assumed to be straight lines mutually orthogonal to each other. Angle error tolerances of the features of fabricated part 100 are customarily specified as runout errors. Runout errors are defined by a baseline along a direction associated with an X, Y, or Z axis and the small departures along both of the other two axis directions. For example, if the baseline is along the Y axis direction the departures may be both along the X axis direction and the Z axis direction. The physical units for specifying small runout are inches in the departure direction divided by inches in the baseline direction, where it is desirable to have a long baseline to measure better runout accuracy. The runout is essentially an angle measured in radians, but it is not the custom of machinists to work in angular units.
The fabrication of the part 100 may involve manipulating the part in the vise on the machine tool table in the X, Y, and Z axis directions so as to find suitable orientations to optimize the fabrication process. A vise with jaws 102 and 103 is used to hold the part 100 shown in FIG. 1A and FIG. 1B. The problem then becomes setting up the fixed vise jaw 102, rather than the part 100, to align with mutually orthogonal X, Y, and Z axes described above. Once the vise is properly positioned, the proper alignment to hold the part 100 with suitable manipulations in the vise, consistent with the mutually orthogonal X, Y, and Z axes described in the engineering drawing is thereby established.
Standard practice setup of the vise typically has a first step to align the vise fixed jaw 102 in the X axis direction defined by table motion X axis direction 104. Alignment of the vise X axis reference surface on fixed jaw 102 can be determined using a measuring probe 112 held by a tool holder 111 in the spindle 110, as shown in FIG. 1B. Runout in the vise X axis direction 140 baseline with departure in Y axis direction 141 at for example the vise points 120 and 121 shown in FIG. 1A, is determined using the measuring probe 112.
Presently the machinist typically uses a mallet to tap the vise producing small corrections to minimize the departure Y axis direction 141 runout. The process of measuring and tapping is repeated to optimize (by minimizing the departure Y axis direction 141 runout) the vise X axis 140 alignment relative to the table motion X axis direction 104. Presently there is no way to tap or otherwise adjust the departure in Z axis direction 142 of vise X axis 140 direction to align with the table X axis direction 104.
It is difficult to precisely align the vise X axis direction 140 by tapping; therefore it is desirable to provide a computer assisted runout correction to the alignment in the event there is a departure error in the Y axis table motion direction 105 causing a runout error between the vise X axis direction 140 and the table motion X axis direction 104.
It is not possible to precisely align the vise X axis direction 140 in the event there is a departure error in the Z axis spindle motion direction 106 causing a runout error in X axis direction 140, therefore it is desirable to provide a computer assisted runout correction to the alignment in the event there is a small Z axis direction 106 departure error between the vise X axis direction 140 and the table motion X axis direction 104.
It is desirable to provide computer assisted runout corrections to the alignment in the event there are small departure errors between the vise Y axis direction 141 and the table Y axis direction 105. Also the table motions in the X, Y, and Z axes are intended to be mutually orthogonal to each other and the vise X, Y, and Z axes are intended to be mutually orthogonal to each other, hence it is also desirable to correct for orthogonal axis errors using runout corrections. Table motions in the X, Y, and Z axes are intended to be mutually orthogonal to each other, and this condition may be determined by a ball-bar test. However, the ball-bar test does not provide information about the vise X, Y, and Z axes.